1. Field of the Invention
The present invention relates to such a toner for developing an electrostatic image that it is excellent in storage stability, and low temperature fixing ability and capable of forming an image of excellent image quality, which contains less fogging.
2. Description of the Related Art
Conventionally, an electrically or magnetically formed latent image in an electrophotographic image forming apparatus is visualized with a toner for electrophotography (may be referred merely as a “toner” hereinafter). In the electrophotography, for example, a static image (latent image) is formed on a photoconductor, followed by developing the latent image with a toner, to thereby form a toner image. The toner image is typically transferred onto a transfer medium, such as paper, followed by being fixed on the transfer member, such as paper. In a fixing step where the toner image is fixed on the transfer paper, a thermal fixing system, such as a heat roller fixing system, and a heat belt fixing system, is widely used because of excellent energy efficiency.
Recently, in the market, there is an increasing need for increased printing speed and energy saving of image forming apparatuses. To this end, desired is a toner having excellent low temperature fixing ability, and capable of providing high quality images. In order to achieve low temperature fixing ability of a toner, it is necessary to lower softening temperature of a binder resin of a toner. When the softening temperature of the binder resin is low, however, so-called offset (may be also referred to as hot offset hereinafter), which is a phenomenon that part of a toner image is deposited on a surface of a fixing member during fixing, and the deposited toner is transferred on a photocopy sheet, tends to occur. Moreover, heat resistant storage stability of the toner is impaired, and so-called blocking, which is a phenomenon that toner particles are fused to each other especially in high temperature environments, tends to occur. Other than those mentioned above, there are problems that the toner is fused onto an inner area of a developing device or a carrier inside the developing device to cause pollution, and filming of the toner tends to be caused on a surface of the photoconductor.
As for techniques for solving these problems, use of a crystalline resin as a binder resin of a toner has been known (see Japanese Patent Application Laid-Open (JP-A) Nos. 2010-077419, 2012-068589, 2011-145587, 2009-229920, and 2008-281884, Japanese Patent (JP-B) No. 3589451, JP-A Nos. 2012-078584 and 2011-113020, and JP-B No. 4999525). Specifically, the crystalline resin can be sharply softened at a melting point of the resin, and the softening temperature of the toner can be lowered to the temperature around the melting point thereof, while maintaining heat resistant storage stability of the toner at the melting point thereof or lower. Therefore, low temperature fixing ability and heat resistant storage stability can be both achieved to a certain degree.
However, there is a problem that pollution of a toner component tends to occur in an image forming apparatus especially due to a continuous use of the image forming apparatus, and image quality tends to be degraded due to aggregations of toner particles.
Meanwhile, as for a toner using a crystalline resin, disclosed is, for example, a toner using, as a binder resin, a crystalline resin obtained by elongating crystalline polyester with diisocyanate (see JP-B Nos. 04-024702 and 04-024703). Moreover, disclosed is a toner using, as a binder resin, a crystalline resin obtained by modifying crystalline polyester with diisocyanate and diol (see JP-A No. 2012-088353). Furthermore, disclosed is a toner using, as a binder resin, a resin obtained by linking crystalline polyester and non-crystalline polyester with diisocyanate (see JP-A No. 2012-042508). Moreover, disclosed is a toner using a crystalline resin having a cross-link structure due to an unsaturated bond containing a sulfonic acid group (see JP-B No. 3910338). Moreover, disclosed is a technology where a ratio of softening temperature and peak temperature of heat of melting, and viscoelastic properties are specified to achieve resin particles having excellent low temperature fixing ability and heat resistant storage stability. These toners can achieve both low temperature fixing ability and heat resistant storage stability for sure, and hot offset resistance thereof is also improved. However, plastic deformability or elongation deformability of a crystalline resin itself cannot be fundamentally removed even by introducing a crosslink structure or a hard segment, such as a urethane bond or urea bond to the crystalline resin. The problem of the pollution of the toner component to the image forming apparatus especially due to continuous use of the image forming apparatus or degradation of image quality due to aggregations of the toner particles has not been solved.